[FIX] sale: Taxes mis-aligned with their column label

Regression introduced with 71c624163c.

If the discount column is not added in the `tbody`
when not part of the group `sale.group_discount_per_so_line`,
the head column should not be added either in the `thead`

opw-653242

addons/sale/views/report_saleorder.xml

@@ -58,7 +58,7 @@
                         <th>Description</th>
                         <th class="text-right">Quantity</th>
                         <th class="text-right">Unit Price</th>
-                        <th class="text-right"><span groups="sale.group_discount_per_so_line">Disc.(%)</span></th>
+                        <th class="text-right" groups="sale.group_discount_per_so_line">Disc.(%)</th>
                         <th class="text-right">Taxes</th>
                         <th class="text-right">Price</th>
                     </tr>